Field-Effect Transistor (FET) switches often introduce undesired voltage transients. For example, an N-channel, enhancement mode, Metal-oxide Semiconductor Field-Effect Transistor (MOSFET) switch is turned on by applying a gate drive signal to the gate of the FET. The gate drive signal makes a voltage transition from a supply level to a level above the analog signal to be switched. When the FET switch turns off, the opposite transition is made. In either case, part of the drive signal transition occurs while the switch FET is on, and part while the switch FET is off. For the ‘off’ part of the transition, the gate-to-drain capacitance of the FET couples into the input node and injects charge into the input, causing a voltage transient. On the other hand, for the ‘on’ part of the drive signal transition, the sum of the gate-to-drain, gate-to-source and gate-to-channel capacitances of the FET couples into the input node and injects charge into the input, causing a voltage transient.
One approach to reducing or eliminating the charge injection is to use a compensating FET and a capacitor. In this approach, the gate drive voltage of the compensation FET and/or capacitor is equal in magnitude to that of the switch FET but opposite in direction. The length of time the FETs are on varies with the input signal level and therefore changes the total amount of charge transfer. Consequently, this compensation technique will be less effective for some voltages than others. Another approach uses a programmable digital-to-analog converter (DAC) in an auto-calibration loop. A zero voltage level is applied to the high impedance input of the DAC. Measurements are then made using the analog-to-digital converter (ADC) of a digital multi-meter while the switch is toggled on and off. A programmable capacitor is adjusted until the reading is zero. This technique is expensive, due to external components, and requires a calibration algorithm. Additionally, the speed of compensation is limited, so high frequency injection is not well compensated.